stylus: Add missing Renderer.render() variant

stylus/stylus.d.ts

@@ -647,6 +647,11 @@ declare module Stylus {
          */
         render(callback: RenderCallback): void;
 
+        /**
+         * Parse and evaluate AST and return the result.
+         */
+        render(): string;
+
         /**
          * Get dependencies of the compiled file.
          */